The present invention relates to a demodulator of a burst communication system using an OFDM (Orthogonal Frequency Division Multiplexing) modulating method and, more particularly, to an OFDM demodulator using a simple timing reproducing circuit.
An OFDM modulating method has been being examined as a modulating method of multimedia communication such as a high-speed wireless LAN. In a high-speed wireless LAN having information transmission speed of tens Mbits/sec, when a conventional modulating method of performing modulation such as QPSK on a single carrier wave, large waveform distortion occurs over a plurality of symbols by a multipath delay wave. On the other hand, in the OFDM modulating method, multicarrier modulation in which an information signal is divided into a plurality of subcarriers is performed, and the waveform distortion caused by the multipath delay wave is reduced by insertion/deletion of a guard interval (GI). Consequently, the method is adapted to a high-speed wireless transmission system.
In a wireless LAN, generally, a signal is transmitted/received in a packet. At the head of a packet used for synchronization of the packet, a preamble for frequency synchronization and symbol timing synchronization is provided.
FIG. 6 is a diagram showing a format of an OFDM burst signal compliant with IEEE802.11 in which the international standard of a wireless LAN is specified. In the diagram, at the head of burst data 23, a preamble 24 for OFDM frequency synchronization and symbol timing synchronization and a preamble 25 for estimating a channel characteristics path are provided.
The frequency synchronization is performed to correct a frequency error of an oscillator of a transmitter/receiver. It is important since the OFDM signal deteriorates by a frequency error more than a conventional modulation signal. The symbol timing synchronization is established for a process of demodulating an OFDM burst signal.
The shorter the preamble 24 for timing/carrier frequency synchronization is, the more it is desirable from the viewpoint of transfer efficiency. Consequently, a plurality of symbols each of which is shorter than the length of the OFDM symbol (generally, about 4 xcexcsec) are arranged. After that, the preamble 25 for estimating the channel characteristic of the OFDM symbol length is disposed.
In FIG. 6, five short symbols are provided as an example. The number of short symbols is properly selected according to the synchronization condition. As the time of the short symbol, time shorter than the time of the OFDM symbol, for example, about xc2xd or xc2xc of the time of the OFDM symbol is generally set.
In the case of using such a short symbol, although there is an advantage such that the process delay time is short, since the symbol cycle is short, an average sample number cannot be obtained in a symbol reproducing circuit and there is a problem such that timing detection accuracy cannot be obtained.
There is also a problem such that since the short symbol is used, a method of detecting a timing of switching the short symbol to a normal OFDM symbol is complicated.
It is an object of the invention to provide an OFDM demodulator using a symbol reproducing circuit capable of easily detecting a preamble for timing/carrier frequency synchronization using short symbols with high detection accuracy.
In order to achieve the object, according to the invention, there is provided an OFDM demodulator for demodulating an orthogonal frequency division multiplexing (OFDM) modulated burst signal, comprising:
a quasi-synchronous detector for converting the OFDM modulated bust signal into two complex baseband signals by quasi-synchronous detection;
a carrier frequency estimating circuit for receiving the complex baseband signals, estimating a carrier frequency, and compensating a carrier frequency error;
a symbol timing estimating circuit for controlling the width of a detection window on the basis of a result of correlation between a preamble signal in the complex baseband signal and a prestored pattern signal and reproducing a timing of the burst signal;
a symbol synchronization processing circuit for establishing symbol synchronization on the basis of an output of the symbol timing estimating means with respect to the complex baseband signal in which the carrier frequency error has been compensated;
an FFT for performing Fourier transform on an output of the symbol synchronization processing circuit to thereby divide the output into signals of respective subcarriers; and
a subcarrier demodulator for demodulating an output of the FFT every subcarrier.